Impact of EHB1 and AGD12 on Root and Hypocotyl Phototropism in Arabidopsis thaliana

Previous findings concerning two proteins involved in gravity and light-induced signal transduction, (AGD12-ADP-RIBOSYLATION FACTOR GTPase-ACTIVATING PROTEIN 12 and EHB1 (ENHANCED BENDING 1)), in Arabidopsis provided evidence for an antagonistic regulation of a gravitropic stimulus on roots and shoots. Both proteins share high C-terminal homology containing a canonical calcium-binding C2-domain, but EHB1 lacks a functional N-terminally located GTP-binding domain. Physiological analysis using loss-of-function mutants under clinostat conditions, where any gravitropic stimulus could be ruled out, provided evidence for diminished phototropism of agd12 mutants, but enhanced phototropism of ehb1-mutants. Data could be confirmed by seedlings overexpressing either AGD12 or EHB1, showing enhanced bending in AGD12 and reduced bending in EHB1 overexpressing seedlings. We conclude that EHB1 and AGD12 are early signaling elements of the gravitropic phototropic bending response, because both of them bind to NPH3 (NON PHOTOTROPIC HYPOCOTYL), which was described as the first element of the phototropic signal transduction chain, directly interacting with phot1. Both proteins affect phototropic bending antagonistically. In contrast to the total incapability of agd12 and ehb1 mutants to respond to increased calcium concentration in response to gravity, agd12 nor ehb1 seedlings differ only gradually in their calcium induced bending response to blue light. This would indicate that EHB1 and AGD12 might be triggered by other signal elements within the phototropism signal transduction chain.